1. Field of the Invention
The present invention relates to methods and apparatus for improving the performance of a receiver and, more particularly, to improving the false alarm rate and detection probability in a receiver employing sensitivity time control.
2. Description of the Prior Art
The use of constant false alarm rate (CFAR) receivers in search and surveillance type radar systems is well known. It is typical for such radar systems to operate in environments where undesired echoes or return signals from adverse weather conditions and surface clutter (sea and/or ground, depending upon the radar application) are substantial. Such undesired echoes are sources of interference which can severely interfere with a radar display and/or overload data processing equipment associated with the radar receiver. At times, such interference may rival the thermal noise levels attributable to the radar receiver.
It is known that both the probability of detection of a desired target and the probability of false alarms (undesired detections erroneously classified as targets) are dependent upon the level of receiver thermal noise and interference caused by adverse weather and surface clutter. The thermal noise attributed to the receiver is typically constant over the operable range of the receiver; however, because of the variation in interference levels due to weather and clutter, it is often desirable to set a constant threshold level for the false alarm rate (i.e., CFAR performance). Typically, such a receiver implementing CFAR senses the magnitude of the radar echoes from noise and clutter and uses this information to set a threshold in order that noise and clutter echoes are substantially rejected at and below the threshold and not erroneously recognized as actual targets. When the magnitude of the radar echoes from noise and clutter is sensed in the near vicinity of the target, this is known as area CFAR. Nonetheless, the threshold may be varied depending on the prevailing conditions of clutter and noise.
There are several known approaches to designing CFAR receivers. A typical design approach includes a mixer used to translate or downconvert the received RF return signal to an intermediate frequency (IF) signal. The mixer is normally coupled to an IF amplifier wherein the signal is amplified before being presented to a detector. The amplifier may be a logarithmic amplifier, as will be explained, or a linear amplifier. Sensitivity time control (STC) may also be implemented in the receiver. In general, sensitivity time control (STC) is a process which causes radar receiver sensitivity to vary with time such that the amplitude associated with the IF return signal is independent of range (i.e., constant as a function of range). STC is desirable due to the fact that search and surveillance radars detect return signals of different amplitudes, typically so great that the dynamic range of a receiver not having STC (i.e., fixed-gain receiver) will be exceeded. When STC is implemented before frequency downconversion from RF to IF then it is known as RF STC, implementation after downconversion is known as IF STC.
As previously mentioned, the amplified and STC responsive signal is then passed on to a detector, such as an AM envelope detector, where the desired target information is separated from the carrier signal. CFAR levels are adjusted either automatically by the computer processing equipment or manually by an operator attentive to the prevailing environmental conditions. There are many other types of design techniques for implementing CFAR, several of which are discussed in detail in the text entitled: "Radar Handbook" by Merrill Skolnick, pp. 3.46-3.53, 2nd ed. (1990).
However, while CFAR is typically necessary in order to avoid processing overload conditions, there are disadvantages and limitations associated with CFAR receivers. For instance, it is known that CFAR reduces the probability of detection, causes a loss in the signal-to-noise (S/N) ratio, and degrades range resolution. Furthermore, certain users of radar systems which implement CFAR, such as Air Traffic Controllers (ATCs) who are viewing the radar output of an airport Surface Movement Radar (SMR) on their displays, may not be as interested in a constant false alarm rate as they might be with a constant probability of detection with a varying false alarm rate (assuming that the worst probability of false alarm is satisfactory).
Generally, it would be desirable to provide a radar receiver capable of implementing CFAR performance, if desired, but which would advantageously provide an improved false alarm rate without substantially degrading the probability of detection, the S/N ratio, or the range resolution associated with the receiver. In fact, it would also be desirable to provide such a radar receiver which achieves the above-mentioned advantages, yet also actually increases the probability of detection.
Specifically, it would be desirable to provide a low cost receiver capable of implementing CFAR which utilizes a logarithmic amplifier and RF STC and which overcomes the limitations associated with degraded probability of false alarms due to the use of RF STC.